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The Increasing Pandemic Potential of H7N9 Avian Influenza

Amesh A. Adalja, MD, FACP, FACEP, FIDSA, March 3, 2017

Of the infectious disease threats that the world faces, avian influenza viruses rank among the most alarming. Recent expanded activity related to the H7N9 virus in China has become increasingly worrisome and underscores the ongoing need to continue preparedness activities against this and other influenza viruses. 


First Appeared in 2013

Human infections with the H7N9 strain of influenza A were first reported in 2013 in China. In the 4 years since, the number of human cases of H7N9—unlike other novel avian influenza viruses—have risen at a relatively fast rate and now number more than 1,200. 

Since 2013, the virus has appeared in seasonal waves, and cases have been strongly linked to poultry exposure, though some familial clustering may have occurred. Approximately one-third of those infected succumb to the virus, with older adults and those with comorbidities being at an elevated risk. In avian species, by contrast, the virus is considered to have low pathogenicity.

Because of the virus’s pandemic potential, a candidate vaccine has been stockpiled by the US government. 


Modern Incarnation

In the latest wave of H7N9 infection, several concerning developments have occurred that have heightened attention to the pandemic potential of this virus, which is now arguably greater than the more lethal and older H5N1 variant that much pandemic preparedness has been focused on. 

Specifically, changes have arisen in the virus’s genetic sequence converting it from low pathogenicity to high pathogenicity in poultry, coupled with concerns regarding the match of the stockpiled virus to currently circulating strains. The 12 million doses of stockpiled vaccines are targeted against the Yangtze River Delta strain of the virus, while new cases are being caused by the Pearl River Delta lineage.


The H7 Mystique

When it comes to avian influenza viruses, those of the H7 variety have made multiple incursions into humans, reaching back as far as 1959. In addition to H7N9, viruses including H7N3, H7N2, and H7N7 have all caused infections in humans. The ability of H7 influenza viruses to spill into humans has definitely been demonstrated on multiple occasions. While H5N1, with its 60%+ mortality rate, rightly garners a lot of attention, H7 flu viruses merit a similar level of concern. 

As the outbreak continues, close attention should be paid to genetic changes to the virus, exposure histories of those infected, antiviral resistance patterns, and vaccine strain match.



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Branswell H. Human cases of bird flu are surging, alarming public health officials. STAT February 28, 2017. Accessed March 1, 2017. 

Branswell H. As bird flu spreads, US concludes its vaccine doesn’t provide adequate protection. STAT March 1, 2017. Accessed March 1, 2017.